The present invention pertains to a composition yielding an abrasion-resistant tintable coating on a thermoplastic or thermoset substrate, such as an ophthalmic lens.
It is known that transparent plastic materials such as polycarbonate ophthalmic lenses or screen face plates are subject to becoming dull and hazy due to scratching and abrasion during use.
Attempts have been made to overcome this problem. The technical solutions proposed in the past, which involved applying a UV-curable coating, generally used a solvent-borne composition which was usually substrate-dependant. That is, the coating compositions were formulated for one specific ophthalmic lens material such as CR-39 or thermoplastics such as polycarbonate. The few compositions found in the literature that were solvent-free or substantially solvent-free were either substrate-dependant or contained a partially hydrolyzed or fully hydrolyzed silane used both for adhesion and for abrasion resistance.
Moreover, coatings for ophthalmic lenses should also be capable of being tinted by incorporating a dye therein. However, abrasion and scratch resistance, on the one hand, and tintability, on the other hand, are often regarded as hardly compatible properties.
Among the solutions proposed to reconcile these properties, U.S. Pat. No. 5,614,321 suggests a curable coating composition comprising colloidal silica, together with a (meth)acrylate compound capable of reacting with said silica, a monomer (preferably an alkoxysilane) bearing (meth)acryloxy groups, a free radical initiator and an organic tintability additive. US 2002/0193479 teaches a composition comprising both an hydrolyzed and a non-hydrolyzed epoxy-functional alkoxy silane, together with a curing agent and an acrylic monomer preferably bearing not more than two acrylic functions. Similarly, U.S. Pat. No. 6,100,313 discloses a composition comprising an epoxy-functional alkoxysilane, a glycidyl ether, a cationic photo-initiator, an acrylic monomer and a free-radical photo-initiator.
It has been observed that compositions that utilize silane monomers, although both tintable and abrasion resistant, are hydrolytically unstable and have a rather short pot-life in a coating machine designed to recirculate the coating, because they absorb moisture from the atmosphere and/or from lenses that are not completely dry (after automatic water wash) during the coating process. This causes an increase in viscosity and produces coating flow defects on coated lenses. The hydrolyzed silane can also yield silica particles in the coating machine, resulting in particulate coating defects.
We have now discovered that it was possible to formulate a silane and silica-free coating composition bases on a specific combination of epoxy and acrylic monomers. This composition, when cured, leads with a good coating yield to a coating which provides excellent adhesion both to thermoset and thermoplastic substrates (especially with the commercial vacuum deposition anti-reflective coatings), which is tintable with an excellent uniformity, abrasion and scratch-resistant and which is hydrolytically stable both in a UV resistant bottle and when used in a coating machine that recirculates the coating.